The term “excavation work device” according to the invention is understood to refer to devices that can be used to transfer the drive power of a drive device to a tool, via a linkage, that is arranged at a linkage. This includes, in particular, auger devices that can be used to make drill holes, and particularly horizontal holes, in the ground. In doing so, thrust or compressive forces and a drive torque are normally transferred from the drive device to the tool, which is in the form of a drill head, via the linkage of the auger device. There are, however, also excavation work devices with which only thrust or tractive forces are transferred to a corresponding tool via the linkage. These include, in particular, those excavation work devices that can be used to expand existing drill holes in the ground or previously laid old lines, or to remove them, and any new pipework can be laid at the same time. Often, the excavation work devices are designed such that they can be used both for creating the auger holes as well as for pulling work, i.e. for expanding an existing hole or an old line for placing new pipework. This makes it possible to initially create a pilot drill hole with the same excavation work device, during which a pilot drill head is operated so as to thrust through the ground until it reaches a target excavation pit and the pilot drill head is replaced in the excavation pit by an expanded head, by means of which the pilot drill hole is expanded while the drill linkage is being pulled out. As an option, a new pipe, which is attached to the expanded head, can be placed into the expanded ground drill hole at the same time as the expanded head.
The term “drive section” according to the invention refers to an attachment or a connecting and/or screw element for a drive, for example a linear motor or a torque motor, of a device, or particularly an excavation work device. The drive section can be, for example, a connecting element screwed onto a drive shaft of a torque motor. The drive can generate the thrust, compressive, and tractive forces as well as a torsional force or torque required for the device to work. The tractive force is required, for example, during the placement of a new pipe for an excavation work device. The transfer of the torque is important, for example, for a ground drilling system in the horizontal drilling sector.
The linkage of such types of excavation work devices normally consists of a plurality of linkage segments that are connected to one another, in stages, depending on the drilling propulsion. The individual linkage segments are connected via a coupling element in that, in addition to the plug couplings, as described in DE19608980 A1, screw connections, in particular, are widely used. The significant advantages of screw connections are the low costs that are associated with their production as well as the possibility of automating the screw-on process that is to be executed, in a simple manner. A significant disadvantage of screw connections, however, is that they often represent the weakest points in the linkage, which is due to the relatively small diameter in the area of the threaded plug of the threaded connection as well as to the large notch effect of the thread itself due to its geometry.
Known types of threads that are used for the linkages of excavation work devices are API threads as well as round threads in accordance with DIN 20400. The service life of such types of thread shapes, however, has proven to be insufficient in the daily operation of excavation work devices.
Therefore, alternative thread shapes have been developed that are specially designed for the specific linkage loads occurring with excavation work devices. DE19803304 A1 discloses a thread connection that is intended to connect the linkage segments of a drill linkage in which the drill linkage is intended to be used for rotary percussion drilling. The essential details of these thread connections are the asymmetrical shape of the thread with different gradient angles of the load-bearing and the non-load-bearing flanks in rotary percussion drilling as well as the provision of a non-threaded lead section. In addition, the non-load-bearing flanks should be formed in a circular arc shape. The threaded connection known from DE19803304A1 has also been shown to be insufficiently robust in daily operation.
EP0324442B1 and U.S. Pat. No. 5,060,740A also disclose asymmetrical threaded connections that are designed for connecting the linkage segments of auger devices. The threaded connections disclosed in these printed documents are characterized in that the root of the thread forms a section of an ellipse. This is supposed to enable a transition from the root of the thread into the flat portion of the flanks of the thread in a manner that is as notch-free as possible.
WO2006/092649A1 also discloses threaded connections for the linkages of auger devices in which the thread root forms a section of an ellipse. The thread shapes disclosed in this printed document may be asymmetrical or symmetrical.
Furthermore, EP0660004A1 discloses self-tapping screws equipped with a thread. In order to reduce the notch effect in the thread root with the screws, the area of the transition of the thread root to the flank ascent is designed with a curved profile that has “a harmonic transition from a tangent, extending axially to the thread base, to an outward sloping flank section, forming an angle to the thread axis.” FIG. 3 in EP0660004A1 shows a threaded profile curve forming a section of a horizontal ellipse in the area of the thread root.